I. Technical Field
The present invention relates to an aluminum alloy hollow extruded material for electro-magnetic forming with excellent pipe expansion properties in electro-magnetic forming.
II. Description of the Related Art
Electro-magnetic forming is a method in which a large current of 10 kA or more, for example, is instantaneously passed through a coil to create a strong magnetic field. Forming of a workpiece (conductor) to be shaped placed in the strong magnetic field is achieved by means of an interaction between an eddy current produced in the workpiece and the magnetic field. For example an aluminum alloy hollow extruded material (pipe) is subject to the electro-magnetic forming with an electro-magnetic forming coil placed therein, resulting in that the aluminum alloy hollow extruded material undergoes outward pipe expansion.
Because having low durability, a previous coil for electro-magnetic forming itself breaks when a high electro-magnetic force is produced. This limits the electro-magnetic force that can be output by the coil. Therefore, as raw materials for aluminum alloy hollow extruded material for electro-magnetic forming, aluminum alloys having moderate strength, such as 6000-series alloys, have been used. For example, JP-A-2010-159005 and JP-A-2010-69927 disclose the pipe expansion of a T1-temper 6000-series aluminum alloy hollow extruded material by electro-magnetic forming. Further, JP-A-2007-254833 and JP-A-2005-105327 disclose a 6000-series aluminum alloy hollow extruded material having excellent pipe expansion formability in electro-magnetic forming.
Meanwhile, in recent years, the durability of coils for electro-magnetic forming has been improved, allowing for the output of higher electro-magnetic force. Therefore, it has been considered to apply the electro-magnetic forming to 7000-series aluminum alloys having higher strength than that of 6000-series aluminum alloys.
Descriptions relating to the pipe expansion of 7000-series aluminum alloy hollow extruded materials are disclosed in JP-A-2010-196089, JP-A-2009-114514, and JP-A-2007-119853, for example. However, the pipe expansion methods in these documents are pipe expansion by pushing a conical die into the pipe or pipe expansion by hydroforming.
Any of the 7000-series aluminum alloy hollow extruded materials described in these documents contains considerable amounts of one or more of Mn, Cr, and Zr and has a fibrous structure as its crystal structure, therefore regarded as having excellent pipe expansion formability. Because being generally used for a structural member, in order to improve SCC resistance (resistance to stress corrosion cracking), a 7000-series aluminum alloy is subject to addition of transition elements such as Cr, Mn, and Zr that causes its crystal structure to be fibrous.